Method of liquid deposition by selection of liquid viscosity and other precursor properties

ABSTRACT

A plurality of liquids, the flow of each controlled by a volumetric flowrate controller, are mixed in a mixer to form a final precursor that is misted and then deposited on a substrate. A physical property of precursor liquid is adjusted by adjusting the volumetric flowrate controllers, so that when precursor is applied to substrate and treated, the resulting thin film of solid material has a smooth and planar surface. Typically the physical property is the viscosity of the precursor, which is selected to be relatively low, in the range of 1-2 centipoise.

RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 09/128,058 filed Aug. 3, 1998, now U.S. Pat. No.6,143,063, which is itself a continuation-in-part of U.S. patentapplication Ser. No. 08/610,330 filed Mar. 4, 1996, now U.S. Pat. No.5,962,085.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the fabrication of integrated circuits, and inparticular to a method of improving the deposition of materials duringthe fabrication of integrated circuits by adjusting a physical propertyof a precursor liquid.

2. Statement of the Problem

The manufacture of integrated circuits entails series of numerous stepsin which thin layers of materials are sequentially deposited, patternedand etched to form the various components of the circuit. Therefore, toutilize a material in an integrated circuit it is necessary to be ableto make a high quality thin film of the material. It is essential thatthe quality must be high not just from a macroscopic viewpoint, but,since the individual components of an integrated circuit aremicroscopic, the film must also be of high quality when examined on themicroscopic level.

One of the problems associated with integrated circuit manufacture isthat it is difficult to make high quality thin films of some materials,particularly complex materials such as layered superlattice materials.Often the best methods known to make these materials are complex,inefficient or unreliable. Also, the methods of deposition of variousmaterials vary, and this lack of uniformity causes interruptions andinefficiency in the overall manufacturing process.

One category of common deposition methods is liquid deposition, in whicha precursor liquid of the material to be deposited is applied to anunderlying substrate. The precursor may be a simple solution of thematerial to be deposited dissolved in a solvent. More typically, theprecursor is a solution of one or more chemical precursor species thatare reacted on the substrate after the precursor liquid is applied.After a coating of precursor liquid has been applied, it is usuallytreated to form a solid layer of desired material.

In general, the quality of the deposited solid layer of materialdepends, among other factors, on the properties of the liquid coating ofprecursor. Some of the many properties of the liquid coating thatinfluence the integrated circuit manufacturing include: theconcentration of material or chemical precursor species in the liquidcoating; the thickness of the liquid coating; and the coverage of theunderlying substrate by the liquid coating. For example, usually goodstep coverage of a substrate by an overlying solid layer is desired. Atthe same time, it is often important for the deposited solid layer ofmaterial to fill all the depressions and steps in the underlyingsubstrate and present a uniformly smooth and planar surface. In currentmanufacturing processes the combination of good step coverage and asmooth planar surface are achieved by using a chemical mechanicalpolishing (“CMP”) process step after formation of the solid film. Thiscan be the source of problems in the manufacturing process because theCMP step not only adds to the complexity and expense of themanufacturing process, but it also causes particle formation. Particlesformed in the CMP process are difficult to entirely remove and canseriously degrade the integrated circuit. A related problem in currentmanufacturing processes is that some otherwise practical depositionmethods cannot be used to deposit layers of certain solid materialsbecause the ensuing CMP process leads to insurmountable problems.

The quality of the applied precursor liquid coating and the resultinglayer of solid material are also influenced by the deposition method andconditions. For example, if a misted deposition method is used to applythe coating of precursor liquid, the flow rate of mist and the particlesize of the mist influence the qualities of the liquid coating. At thesame time, the physical qualities of the precursor liquid influence theefficacy of a given deposition apparatus to apply a liquid coating ofdesired qualities on the substrate. A problem in current manufacturingmethods is that otherwise practical deposition methods cannot be used todeposit layers of certain materials because the quality of the precursorliquid is not suitable for operation with a given type of depositionapparatus.

3. Solution to the Problem

The invention solves the problems by selectively and systematicallyadjusting the physical properties of the precursor liquids to replaceone or more conventional integrated circuit processing steps. In onepreferred embodiment, the viscosity or other related physical propertyis selected so that the precursor fills depressions in the substrate andat the same time provides a smooth planar surface, thereby eliminatingthe need for a CMP process. In another embodiment a first precursorhaving a first viscosity or other related physical property is depositedfollowed by the deposition of a second precursor having a secondviscosity or other related physical property. In this embodiment,preferably both the first and second precursors are precursors forforming essentially the same predetermined material.

The invention also solves the above problems by providing a method offabricating a thin film of a solid material in which a physical propertyof a liquid precursor is changed during the step of depositing theprecursor on a substrate. The physical property can be changed in stepsor continuously. The liquid precursor can be dried between the changingsteps or only after both the original precursor and the changedprecursor are applied.

The invention also solves the above problems by providing an apparatusfor forming a thin film of a solid material having two liquid sourcesand a means for mixing the two liquids in a continuous and controlledmanner just prior to the step of depositing the liquids.

The invention provides a method of fabricating a thin film of solidmaterial having a planarized thin film surface, the method comprisingsteps of: providing a substrate having a substrate surface includingnon-planarized areas; providing a precursor liquid for forming the layerof solid material, the precursor liquid having a viscosity of 5centipoise or less; applying the precursor liquid to the substrate; andtreating the precursor liquid on the substrate to form the thin film ofsolid material having the planarized thin film surface. Preferably, theviscosity does not exceed 2 centipoise, and most preferably theviscosity is between 1 centipoise and 2 centipoise. Preferably, the thinfilm of solid material comprises a metal oxide, such as a layeredsuperlattice material, silicon dioxide, or a silicon glass. Preferably,the precursor liquid includes a compound selected from the groupconsisting of alkoxides, carboxylates, and combinations thereof.Preferably, the precursor includes a metal compound, and preferably themetal compound is selected from the group consisting of metal2-ethylhexanoates and metal 2-methoxyethoxides. Preferably, theprecursor liquid comprises a solvent and the solvent is selected fromthe group consisting of alcohols, aromatic hydrocarbons, and esters.Preferably, the step of applying comprises spin-coating a thin film ofthe precursor liquid on the substrate. Alternatively, the step ofproviding a substrate comprises placing a substrate inside an encloseddeposition chamber; and the step of applying comprises the steps of:producing a mist of the precursor liquid, and flowing the mist throughthe deposition chamber to form the precursor liquid on the substrate.

The invention also provides a method of fabricating a thin film of solidmaterial comprising steps of: providing a substrate and a precursorliquid suitable for forming a thin film of the solid material uponapplication to a substrate and treatment; applying the precursor liquidto the substrate; changing a physical property of the precursor liquidsubsequent to the step of applying; subsequent to the step of changing,continuing to apply to the substrate the precursor liquid having thechanged physical property; and treating the precursor liquid on thesubstrate surface to form the thin film of solid material. Preferably,the method further includes the step of drying the precursor liquid onthe substrate between the steps of applying and continuing to apply.Preferably, the steps of changing and continuing to apply are repeatedat least once. Preferably, the step of changing is performed at the sametime as the step of continuing to apply. Preferably, the step ofchanging is performed continuously for a predetermined time. Preferably,the physical property is viscosity, the surface tension between theprecursor liquid and the substrate surface, density, heat capacity, theheat of vaporization, or vapor pressure.

In a further aspect, the invention also provides a method of fabricatinga thin film of solid material comprising the steps of: providing asubstrate; providing a precursor liquid; adjusting a physical propertyof the precursor liquid while applying the precursor liquid to thesubstrate; and treating the precursor liquid on the substrate surface toform the thin film of solid material.

In another aspect, the invention provides a method of fabricating a thinfilm of solid material comprising steps of: providing a substrate, afirst liquid, and a second liquid; flowing the first liquid through afirst fluid flow controller and the second liquid through a second fluidflow controller; mixing the first liquid with the second liquid inamounts determined by the first and second fluid flow controllers toform a precursor liquid; applying the precursor liquid to the substrate;and treating the precursor liquid on the substrate surface to form thethin film of solid material. Preferably, the method further includes thestep of adjusting the first fluid flow controller to change the flow ofthe first liquid through the first fluid flow controller during the stepof applying. Preferably, the at least one of the fluid flow controllersis a volumetric flowrate controller. Preferably, the thin film of solidmaterial comprises a metal oxide, such as a layered superlatticematerial, silicon dioxide, or a silicon glass. Preferably, the precursorincludes any of the compounds and solvents mentioned above. Preferably,the step of treating comprises a process selected from the groupconsisting of: exposing to vacuum, exposing to ultraviolet (UV) and/orinfrared (IR) radiation, electrical poling, drying, heating, baking,rapid thermal processing, and annealing.

The invention also provides apparatus for fabricating a thin film ofsolid material, the apparatus comprising: a source of a first liquid, asource of a second liquid, and a mist generator; a first volumetricflowrate controller connected between the source of a first liquid andthe mist generator and a second volumetric flowrate controller connectedbetween the source of a second liquid and the mist generator; adeposition chamber in fluidic communication with the mist generator; andan exhaust assembly in fluidic communication with the depositionchamber. Preferably, the apparatus further includes a mixing chamberbetween the volumetric flowrate controllers and the mist generator.Preferably, the first liquid is a solvent and the second liquid is acompound selected from the group consisting of alkoxides, carboxylates,and combinations thereof.

The invention not only permits relatively simple changes in depositionparameters to replace relatively complex conventional integratedfabrication process, but results in integrated circuit materials havingbetter electrical properties. Preferably, the variations from flatnessof the surface of a thin film made according to the invention, prior topatterning, are 5% or less of the total thickness of the layer. Mostpreferably, the variations from flatness are 3% or less of the totalthickness, and in routine uses of the invention there often are novariations more than 1% of the total thickness. Numerous other features,objects and advantages of the invention will become apparent from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of an intermediate stage of aportion of an integrated circuit fabricated according to the invention;

FIG. 2 shows a cross-sectional view of a portion of a prior artintegrated circuit in which an additional layer is deposited on apartially formed integrated circuit as shown in FIG. 1 by a conventionalprocess resulting in a depressed region in the layer surface;

FIGS. 3 and 4 show cross-sectional views of further stages in thefabrication process according to the invention of the portion of theintegrated circuit of FIG. 1;

FIG. 5 depicts a cross-sectional view of a portion of a prior artintegrated circuit in which a lower layer is patterned and etched andthen covered with an upper layer in a conventional process, resulting inan upper layer conforming to the shape of the lower layer, having raisedregions and a depressed region;

FIGS. 6 and 7 show cross-sectional views of further stages in anotherembodiment of a fabrication process according to the invention having alower layer as in the prior art integrated circuit of FIG. 5, but inwhich the upper layer was formed using a method according to theinvention, resulting in an upper layer having a substantially planarsurface;

FIG. 8 shows a cross-sectional view of a portion of a prior artintegrated circuit showing an exemplary nonvolatile ferroelectric randomaccess memory (“FeRAM”) cell or dynamic random access memory (“DRAM”)cell fabricated using the prior art;

FIG. 9 depicts a cross-sectional view of FeRAM or DRAM cell fabricatedaccording to the invention;

FIG. 10 depicts cross-sectional view of a prior art FeRAM or DRAM cellin which a metal oxide dielectric layer of a capacitor was formed usingprior art resulting in depressed regions in the metal oxide layer andlayers above it;

FIG. 11 depicts a cross-sectional view of a portion of a DRAM or FeRAMintegrated circuit in which the metal oxide dielectric layer of thecapacitor and an upper interlayer dielectric layer (ILD) were depositedusing the method of the invention, resulting in smooth and planarshapes; and

FIG. 12 illustrates a liquid source deposition system according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

1. Overview

In a misted deposition process, a precursor liquid for a solid materialis prepared, a mist is generated from the liquids, and the mist isflowed through a deposition chamber where it is deposited on a substrateto form a liquid coating of the mist on the substrate. The coating andsubstrate are then treated by such processes as UV and/or IR curing,evaporation in a vacuum, and baking, and then annealed to form a layerof the desired solid material. Basic misted deposition apparati andprocesses are described in detail in U.S. Pat. No. 5,456,945 issued Oct.10, 1995 to McMillan et al., U.S. Pat. No. 5,759,923 issued Jun. 2, 1998to McMillan et al., and U.S. patent application Ser. No. 08/971,799,which are incorporated by reference as though fully disclosed herein, aswell as in numerous other publications, so they will only be describedherein sufficiently to describe the invention.

As is conventional in the art, in this disclosure, the term “substrate”is used in a general sense where it includes one or a number of layersof material, such as layer 14 in FIGS. 1-7, on which another layer ofsolid material may be deposited, and also in a particular sense in whichit refers to a wafer 12, generally formed of silicon, gallium arsenide,glass, ruby, or other material known in the art, on which the otherlayers are formed. Unless otherwise indicated, in this specification, itmeans any integrated circuit layer on which a thin film material isdeposited.

Terms of orientation, such as “above”, “top”, “upper”, “below”,“bottom”, and “lower” herein mean relative to the substrate 12, 102, 381in FIGS. 1, 9, and 11, respectively. That is, if a second element is“above” a first element, it means it is farther from the substrate12,102 or 381; and if it is “below” another element, then it is closerto the substrate 12, 102 or 381 than the other element. The longdimension of substrate 102 defines a plane that is considered to be a“horizontal” plane herein, and directions perpendicular to this planeare considered to be “vertical”.

The precursor liquid solution is preferably obtained by preparingprecursors for each element using alkoxide chemistry, carboxylic acidchemistry, or other wet chemistry techniques, which lead to a commonsolvent. Preferred solvents are xylenes, n-butyl acetate, n-octane, and2-methoxyethanol. The solution having that common solvent is preferablyused as the sole source for the entire deposition process. However, theinvention also contemplates using multiple precursor sources inparallel. In particular, other sources may be used in parallel fordoping or modifying the final desired solid material. For example,another source may be used for doping silicate glass with boron.

For solid materials comprising metal oxide compounds, precursor liquidsgenerally include a metal compound in a solvent, such as sol-gelprecursor formulations, which in general are comprised ofmetal-alkoxides in an alcohol solvent, and metallorganic precursorformulations, sometimes referred to as MOD formulations, which ingeneral comprise a metal-carboxylate formed by reacting a carboxylicacid, such as 2-ethylhexanoic acid, with a metal or metal compound in asolvent, combinations thereof, as well as many other precursorformulations. The preferred solvents include methyl ethyl ketone,isopropanol, methanol, tetrahydrofuran (THF), xylene, n-butyl acetate,hexamethyl-disilazane (HMDS), octane, 2-methoxyethanol, and ethanol. Aninitiator, such as methyl ethyl ketone (MEK), may be added just beforemisting. A more complete list of solvents and initiators as well asspecific examples of metal compounds are included in U.S. Pat. No.5,614,252 issued Mar. 25, 1997 to McMillan et al.

The term “physical property” means any property that can be measuredmacroscopically and characterizes the physical state of the material,such as viscosity, wetting property, i.e., how effectively the precursorwets the substrate to which it is to be applied, surface tension,density, heat capacity, heat of vaporization, vapor pressure, etc. Itdoes not include properties that only can be determined by microscopicmeasurements, such as the stoichiometry of a chemical compound.

The term “planarized” means that a surface is flat within 5% of itsthickness. That is, in a planarized layer, any variations from flatnessin the surface of a layer, e.g., depressions or elevations, are 5% orless of the total thickness of the layer. Thus, “non-planarized” meansthat on the surface of a layer there is at least one elevation ordepression that rises or falls by an amount that is more than 5% of thethickness of the layer.

The term “thin film” herein means a thin film of a thickness appropriateto be used in an integrated circuit. Such thin films are less than 1micron in thickness, and generally are in the range of 20 nanometers(nm) to 500 nm. It is important to distinguish this term from the sameterm, i.e., “thin film”, as used in essentially macroscopic arts, suchas optics, where “thin film” means a film over 1 micron, and usuallyfrom 2 to 100 microns. Such macroscopic “thin films” are hundreds tothousands of times thicker than integrated circuit “thin films”, and aremade by entirely different processes that generally produce cracks,pores, and other imperfections that would be ruinous to an integratedcircuit but are of no consequence in optics and other macroscopic arts.

The term “mist” as used herein is defined as fine drops or particles ofa liquid and/or solid carried by a gas. The term “mist” includes anaerosol, which is generally defined as a colloidal suspension of solidor liquid particles in a gas. The term mist also includes a fog, as wellas other nebulized suspensions of the precursor liquid in a gas. Sincethe above terms and other terms that apply to suspensions in a gas havearisen from popular usage, the definitions are not precise, overlap, andmay be used differently by different authors. For example, “vapor” whenused in its technical sense, means an entirely gasified substance, andin this sense would not be included in the definition of “mist” herein.However, “vapor” is sometimes used in a non-technical sense to mean afog, and in this case would be included in the definition of “mist”herein. In general, the term aerosol is intended to include all thesuspensions included in the text Aerosol Science and Technology, byParker C. Reist, McGraw-Hill, Inc., New York, 1983. The term “mist” asused herein is intended to be broader than the term aerosol, andincludes suspensions that may not be included under the terms aerosol,vapor, or fog.

The term “electrical” herein, when referring to either the “electricalacceleration” or the “electrical charging” of the mist particles isintended to include such aspects based on either electrostatic orelectromagnetic principles or both.

The utility of the invention compared to the prior art is illustratedwith the aid of FIGS. 1-7. FIG. 1 shows a cross-sectional view of anintermediate stage of a generalized fabrication method of a portion 10of an integrated circuit. As depicted in FIG. 1, a layer 14 is formed onan underlying substrate 12 of portion 10, and then a layer 16 is formedon layer 14. Layer 16 is patterned and etched to form an opening 18.Preferably, layer 14 is a metal, such as platinum, and layer 16 is aninsulator, such as spin-on-glass (SOG). FIG. 2 shows a portion 15,similar to portion 10, in which a layer 20 is formed on layer 16 and inopening 18. The prior art method used to form layer 20 results in layer20 conforming to opening 18 and having a depressed region 26. The priorart method uses a precursor liquid for layer 20 having a relatively highviscosity value, for example, 7-8 centipoise, and results in formationof a nonplanar surface as depicted in FIG. 2. In contrast, FIGS. 3 and 4shows further stages in the fabrication of portion 10, in which a layer30 is formed by a method according to the invention using a precursorliquid having a relatively low viscosity value, for example, 1-2centipoise.

According to the invention, either a one-step process, a multi-stepprocess, or a continuously variable process may be used to create theintegrated circuit portion 10 shown in FIG. 4. Preferably, the amulti-step process utilizing two steps is used. In this two-stepprocess, a first precursor is deposited to form the layer 27 which fillsin the opening 18. The first precursor has a viscosity value of 5centipoise or less, preferably less than 3 centipoise, and mostpreferably 2 centipoise or less. Preferably, layer 27 is dried, and thena second precursor is deposited to form layer 27. The second precursorpreferably is formulated to form essentially the same material as thematerial of layer 27, but the viscosity is adjusted to a viscosityhigher than 5 centipoise, and preferably 7 centipoise or higher. In FIG.3, the first layer 27 is shown as only just filling the opening 18 andis not distributed on surface 19, however, the invention alsocontemplates that a small amount of the precursor that is used to formlayer 27 will stick to surface 19 and, after drying, a very thin layerof the desired predetermined material 27 will remain on surface 19. Inany case, after the deposition and drying of layer 28, a dried film ofdesired predetermined material within a desired predetermined thicknessrange covers integrated circuit portion 10. The dried films 27 and 28are then annealed to form layer 30. In the annealing process the twofilms 27 and 28 merge to form the final thin film of desiredpredetermined material 30. In the one-step process the step of FIG. 3 iseliminated, and a precursor having a viscosity value of 5 centipoise orless, preferably less than 3 centipoise, and most preferably, 2centipoise or less, is deposited then dried and annealed to form layer30 in one deposition step. In both embodiments, the method of theinvention results in eliminating or significantly reducing anydepression in the layer 30 corresponding to opening 18. Layer 30 in FIG.4 serves the same integrated circuit function as layer 20 in FIG. 2.Typically layer 30 comprises the same solid material as layer 20, or itmay be different even though it serves the same function in theintegrated circuit. The important point is that the viscosity or someother physical property or properties of the precursor liquid used toform layer 30 was selected to allow layer 30 to fill completely theopening 18.

A feature of the invention is that it has been found that good resultscan be obtained without a drying step between the use of precursorshaving different physical properties. In particular, the physicalproperties of the precursor can be changed in a continuous and gradualmanner from one property to another. For example, as the opening 27 isfilled, the viscosity can be gradually changed from about 2 centipoiseto about 7 centipoise, while continuing the deposition. The increasedviscosity precursor then sticks and accumulates on the substrate,including over the layer 27. After deposition of both layers 27 and 28,they are dried and annealed to form layer 30. The invention contemplatesthat other multiple step processes, between the two-step process and thecontinuous process, such as three or more discrete steps, may be used.

FIG. 7 is a cross-sectional view of a portion 40 of an integratedcircuit, in which a layer 20 is patterned, etched and then covered witha layer 41. Layer 20 may comprise a solid material that is not amenableto deposition using the inventive method. For example, layer 20 maycomprise a metal electrode or local interconnect layer that canpractically be formed only by sputtering of a target. As a result, adepression 26 as in FIG. 2 characterizes the surface of layer 20 in FIG.5. When layer 41 is deposited, using a prior art method, layer 41conforms to the shape of layer 20, and raised regions 42 and 44 and adepressed region 46 result. In contrast, FIG. 7 shows that if layer 50is deposited according to the invention to cover the same layer 20 ofFIG. 5, then the resulting surface of layer 50 is substantially planar,and the raised regions 42 and 44 and the depressed region 46 of FIG. 6are eliminated or significantly reduced. Again, FIGS. 6 and 7 illustratea two-step process of forming the layer 50 in which layer 48 is formedby a low viscosity precursor and layer 49 is formed by a higherviscosity precursor, and then layer 50 is formed in the anneal process.Similarly to the discussion above, the step of FIG. 6 can be eliminatedand layer 50 can also be formed by a one-step deposition process.Alternatively, a continuously variable process or a multi-step processutilizing three or more steps may be used.

During, after, or both during and after applying the precursor liquid,the liquid coating is treated to form a thin layer of solid material 30,50 on the substrate. In this context, “treated” means any one or acombination of the following: exposed to vacuum, UV and/or IR radiation,electrical poling, drying, and heating, including baking and annealing.In the preferred embodiment, UV radiation is optionally applied to theprecursor solution during deposition. The UV radiation is preferablyalso applied after deposition. After deposition, the liquid coating ispreferably exposed to vacuum for a period, then is baked and thenannealed. The preferred process of implementing the invention includes:deposition of the misted precursor liquid directly on the substrate;dissociation of the organics contained in the precursor that do not formpart of the desired solid material; and removal of the solvent andorganics or other fragments primarily after the liquid coating is on thesubstrate. However, in another aspect the invention also contemplates aprocess in which the final desired chemical compound or an intermediatecompound is separated from the solvent and organics before the liquidcoating is applied, and the final desired chemical compound or anintermediate compound is deposited on the substrate. In both aspects,preferably one or more bonds of the precursor pass through to the finalsolid layer. After the formation of the desired layer of solid material30, 50, the integrated circuit is continued to completion. Thisgenerally includes patterning of the layer 30, 50.

In the discussion above, the material 30, 50 is preferably a complexcompound, which is defined as a compound containing at least two metals.Examples of such compounds are the layered superlattice materials andthe ABO₃ type perovskites. Preferably, the layer 30, 50 forms a portionof an active component of the integrated circuit. The term “active” whenapplied to an electronic element in an integrated circuit, means anelement that alters its state in response to some electrical stimulus.This is to distinguish elements that form important functions in theelectrical circuit, and therefore must be of high and reproduciblequality, from elements that are removed from the integrated circuitbefore its completion or which merely serve to separate portions of thecircuit or act as a packaging element for the circuit, which elements donot need to be of nearly as high and reproducible quality.

2. Detailed Description

In FIG. 8 there is shown a cross-sectional view of an exemplarynonvolatile ferroelectric random access memory (“FeRAM”) cell or dynamicrandom access memory (“DRAM”) cell 100 fabricated using the prior art.FIG. 9. depicts an FeRAM or DRAM cell 200 fabricated according to theinvention. The general manufacturing steps for fabricating integratedcircuits containing MOSFETs and memory capacitor elements are describedin U.S. Pat. No. 5,466,629 issued Nov. 14, 1995 to Mihara et al., andU.S. Pat. No. 5,468,684 issued Nov. 21, 1995 to Yoshimori et al., whichare hereby incorporated by reference as if fully disclosed herein.General fabrication methods have been described in other referencesalso. Therefore, the elements of the circuits in FIGS. 8-11 will besimply identified here. For the sake of clarity, identical elementsdepicted in FIGS. 8-11 are identified with the same reference numerals.

In FIG. 8, a field oxide region 104 is formed on a surface of a siliconsubstrate 102. A source region 106 and a drain region 108 are formedseparately from each other within silicon substrate 102. A gateinsulating layer 110 is formed on the silicon substrate 102 between thesource and drain regions 106 and 108. Further, a gate electrode 112 isformed on the gate insulating layer 110. Source region 106, drain region108, gate insulating layer 110, and gate electrode 112 together form aMOSFET 114.

A first interlayer dielectric layer (ILD) 116 made of BPSG(Boro-phospho-silicate glass) is formed on substrate 102 and field oxideregion 104. An adhesion layer 118 is formed on ILD 116. The adhesionlayer 118 is made of, for example, titanium, and typically has athickness of 20 nm. Adhesion layers, such as titanium, enhance theadhesion of the electrodes to adjacent underlying or overlying layers ofthe circuits.

As depicted in FIG. 8, a bottom electrode layer 122 made of platinum andhaving a thickness of 200 nm is deposited on adhesion layer 118. Adielectric thin film 124 is formed on bottom electrode layer 122.Preferably, dielectric thin film 124 comprises a layered superlatticematerial. A top electrode layer 126, made of platinum and having athickness of 200 nm, is formed on the thin film 124. Electrode layers122 and 126 together with thin film 124 form memory capacitor 128.

An electrically conductive diffusion barrier layer 130 is deposited ontop electrode layer 126. Diffusion barrier layer 130 has a thickness inthe range of 20 to 200 nm, preferably in the range of 20 to 50 nm. Layer130 could also represent an adhesion layer, or it might not be included.Layers 118,122,124,126, and 130 are patterned, in as few as twopatterning process steps, to form memory capacitor 128.

A second ILD 136 made of NSG (Nondoped silicate glass) is depositedusing a prior art method to cover ILD 116 and memory capacitor 128. APSG (Phospho-silicate glass) film or a BPSG film could also be used inlayer 136. As depicted in FIG. 8, ILD 136 conforms generally to theshape of the substrate surface on which it is formed, so that thesurface of ILD 136 has a raised region corresponding to the stack ofpatterned layers forming memory capacitor 128.

ILD 136 is patterned to form wiring holes 142, 144, 146, and 148 forelectrical contacts to MOSFET 114 and ferroelectric memory capacitor128. A metallized wiring layer is deposited, typically using asputtering method, and etched to form local interconnect (“LI”) layersLi 152,154,156, and 158. The metallized wiring layer deposited to formLI 152,154,156 and 158 preferably comprises Al—Si—Cu—Ti with a thicknessof 500-900 nm. The shape and relative vertical locations of these localinterconnects also conform to the shape of the substrate on which theywere applied.

In FIG. 9, ILD 236, corresponding to ILD 136 of FIG. 8, was depositedaccording to the invention. As a result, the liquid coating of precursorfor ILD 236 does not conform closely to the shape of ILD 116 and stackedcapacitor 128. Instead, the liquid coating tends to fill completely theopenings and depressions presented by the substrate surface. The resultis that ILD 236 presents a substantially smooth and planar surfacebefore it is patterned and etched. Preferably, the variations fromflatness of the surface 235 of ILD 236, prior to patterning, are 5% orless of the total thickness of the layer. Most preferably, thevariations from flatness are 3% or less of the total thickness, and inroutine uses of the invention there often are no variations more than 1%of the total thickness. As discussed above with respect to FIGS. 1-7,ILD 236 may be formed by either a one-step or a two-step process. Itshould be noted that memory cell 200 of FIG. 9 is only one of numerouscells in integrated circuit 210 having raised and depressed regions onthe surface of a wafer processed using the method of the invention.Because ILD 236 is substantially planar, the metallized wiring layerdeposited to form LI 252, 254, 256, and 258 is also substantiallyplanar. Since the Li layers are typically sputter-deposited and tend toconform to the substrate on which they are deposited, they showdepressed regions corresponding to holes 242, 244, 246, and 248 eventhough long-through sputtering (LTS) or collimated sputtering is used inan attempt to fill them in. Nevertheless, the top surface 257 of theportion of integrated circuit 210 lies substantially in one plane.

With the aid of FIGS. 10 and 11, the utility of using the method of theinvention to deposit a metal oxide dielectric layer of a capacitor isshown. In FIG. 10, depicting prior art, the FeRAM or DRAM cell 380 isformed on a silicon substrate 381, and includes field oxide areas 382,and two electrically interconnected electrical devices, a transistor 383and a memory capacitor 384. Transistor 383 includes a source 385, adrain 386, and a gate 387. Capacitor 384 includes first electrode 388,metal oxide dielectric thin film layer 389, and second electrode 390.ILD 391 separates the devices 383 and 384, except where drain 386 oftransistor 383 is connected to first electrode 388 of capacitor 384. ILD392 is formed to cover ILD 391 and memory capacitor 384.

Since the layer or layers of electrode 388 are typically sputterdeposited, the shape of electrode 388 practically always has a depressedregion corresponding to the opening to drain 386. When metal oxidematerial is deposited on electrode 388 using prior art methods to formdielectric layer 389, the shape of dielectric layer 389 also has adepressed region, corresponding to the depressed region of electrode388. Similarly, electrode 390 and ILD 392 exhibit a depressed region 396mirroring the depressed regions below. In the prior art, the only way toeliminate these depressed regions was to make one layer much thickerthan desired, then use a CMP process to planarize it.

In FIG. 11, on the other hand, metal oxide dielectric layer 489 isdeposited on electrode 388 using the method of the invention. As aresult, the top surface of dielectric layer 489 is smooth and planar.Therefore, electrode 490 and ILD 492 are also smooth and planar abovethe depressed region of electrode 388. Furthermore, when ILD 492 isdeposited according to the invention, its surface is uniformly planar,having no depressed region above the MOSFET 383, which is lower thanmemory capacitor 484. As a result, the top surfaces of Li 494 and 495are substantially planar, although small depressions corresponding towiring holes 496 and 497 may persist. Again, layer 489 may be formed byeither a one-step, a multi-step process utilizing two or more steps, ora continuously variable process as discussed above.

FIG. 12 depicts a block diagram of a preferred misted depositionapparatus and process for forming an integrated circuit in accordancewith the invention. The apparatus 500 includes: a first source of aliquid, i.e., pressurized reservoir 514, a second source of a liquid,i.e., pressurized reservoir 534; a gas system 572; volumetric flowratecontrollers 515 and 535; a mixer 540; a mist generator 544; a depositionchamber system 520; an ultraviolet and infrared heating system 522; asubstrate heater 518; an exhaust system 523, and mist electricalacceleration power sources 559 and 564.

As indicated in the Overview, above, in a misted deposition process, asubstrate is first prepared for deposition by pretreatment. Here,“pretreatment” preferably comprises exposure to UV radiation, but mayalso include exposure to IR radiation, a bake at a temperature between150° C. and 900° C., and/or exposure to vacuum. A precursor liquid for amaterial, such as an oxide ILD layer or a metal oxide dielectric layer,is prepared, a mist is generated from the precursor liquid, and the mistis flowed through a deposition chamber where it is deposited on asubstrate to form a liquid coating on the substrate. The liquid coatingand substrate are then treated by UV and/or IR curing, evaporation in avacuum, and/or baking, and then annealed to form a layer of the desiredsolid material. The flow of the precursor liquid through the system isbriefly described, as follows.

To begin the process, a first liquid is inserted in pressurizedreservoir 514 and a second liquid is inserted in pressurized reservoir534. The first and second liquids may be any of the precursor liquids,solvents, or initiators discussed above. The deposition chamber system520 is being pumped down to a partial vacuum of about 70 Torr belowatmospheric pressure. Gas system 572 provides pressurized gas,preferably dry nitrogen or other inert gas, via lines 574 and 576,respectively, to precursor reservoirs 514 and 534, respectively, atsufficient pressure to drive the fluid from reservoirs through theirrespective volumetric flowrate controllers 515 and 535, respectively.Pressurized reservoirs 514 and 534 are connected to volumetric flowratecontrollers 515 and 535, respectively, via lines 582 and 584,respectively, and volumetric flowrate controllers 515 and 534,respectively, are connected to precursor mixer 540 via lines 586 and588, respectively. Precursor mixer 540 is connected to mist generator544 via line 589. Gas line 578 is connected between gas source 572 andmist generator 544.

As known in the fluid control art, volumetric flowrate controllers 515and 535 are electronic devices that accurately passes a selected mass ofliquid. Unlike a valve, the flow of liquid through a volumetric flowratecontroller does not depend on the pressure of the fluid flow line, theviscosity of the liquid, or the numerous other parameters that canaffect fluid flow. The volumetric flowrate controllers 515 and 535 arecapable of accurately controlling flow of a liquid to within 2% of theselected flow rate. Preferably, the volumetric flowrate controllers 515and 535 are both a controller model No. LV410 manufactured by ESTEC (aJapanese Corporation) and distributed in the USA by Horiba Instruments,Inc. of San Jose, Calif. This volumetric flowrate controller can controlthe flow of precursor into mist generator 544 from about 0.05 ccm (cubiccentimeters per minute) essentially up to 1 ccm. The volumetric flowratecontrollers 515 and 535 permits minute adjustments of the flow of thefirst and second liquids to mixer 540 and into mist generator 544 andalso permits the same flow to be repeated in later runs. The use of avolumetric flowrate controllers 515 and 535 are very important to obtainrepeatable deposition rate, repeatable viscosities, repeatable wettingand sticking properties, as well as other physical properties of theprecursor.

Mist generator 544 may use one of several methods suitable forgenerating mists from liquids; for example, ultrasonic misting andventuri misting. Preferably, a venturi mist generator is used.Pressurized gas flows through gas line 578 to mist generator 544, withthe gas pressure in line 578 automatically controlled. Preferably, thispressure is between 40 pounds per square inch (psi) (2.76×10⁵ Pa) and 80psi (5.52×10⁵ Pa), and most preferably about 60 psi (4.14×10⁵ Pa).Preferably, the gas is a mixture of an inert gas, such as dry nitrogen,and an easily ionized gas, preferably oxygen or carbon dioxide, and mostpreferably oxygen. The oxygen is added to enhance the charging of themist. The oxygen ionizes readily, and, since, in a gas at roomtemperature the gas particles are continually colliding, assists intransferring charge to the liquid mist droplets. Preferably, the gas is1% to 15% oxygen in volume, and most preferably 5% to 10%. In thepreferred embodiment process, a gas mixture of 95% dry nitrogen and 5%oxygen was used.

To begin the deposition process, volumetric flowrate controllers 515 and535 are adjusted to provide the desired precursor to mixer 540 andthence to mist generator 544. For example, in one preferred embodimentan alkoxycarboxylate strontium bismuth tantalate (SBT) precursor isprovided as the first liquid and THF is provided as the second liquid,and by adjusting flow controllers 515 and 535 sufficient THF is mixedwith the SBT precursor in mixer 540 to provide a precursor withviscosity of about 2 centipoise to the mist generator 544. Hexane and/orpentane may also be used instead of THF. A mist is generated in mistgenerator 544 and flows into deposition chamber system 520 via conduit549. The deposition takes place near atmospheric pressure. The pressureis automatically controlled. Preferably, the gas/mist flow through thesystem from line 589 through mist generator 544 is between 3 liters perminute and 8 liters per minute, and preferably about 5 liters perminute.

A power line 557 runs to mist generator 544 from power generator 559 tocharge the mist, preferably by using corona discharge. The voltageapplied in mist generator 544 is automatically controlled by powergenerator 559. The charged particles are accelerated in depositionchamber system 520 by means of a voltage applied from power source 564via electrical cable 566. The acceleration voltage is automaticallycontrolled. Additional oxygen or carbon dioxide may also be added ifneeded to assist in charging the mist.

In the preferred embodiment corresponding to the two-step processdiscussed in reference to FIGS. 3 and 4, after sufficient deposition hastaken place to fill opening 18, the deposition is temporarily stopped byturning off flow controllers 515 and 535 and venting mist generator 544to a precursor dump. The deposited precursor is dried in situ byapplying a low vacuum and curing at about 150° C. for from 1 minute to 3minutes and then at approximately 260° C. for 3 minutes to 5 minutes,preferably using UV and/or IR radiation as the heat source, to createlayer 27. By “in situ” is meant that the substrate is not removed fromthe deposition chamber during this process. Then, the flow controller535 remains off while flow controller 515 is turned on, and once a goodflow of mist to the mist dump begins, the mist is once again directed todeposition chamber 520 to deposit the layer 28. After a suitable thicklayer of the first liquid is deposited, the deposition is stopped againand layer 28 is formed by drying in situ as before. As an example of thecontinuously variable process discussed with respect to FIGS. 3 and 4above, the deposition is not stopped after the opening 18 is filled, butinstead flow controller 535 is gradually closed to cause the viscosityto increase from about 1-2 centipoise to about 7-8 centipoise.Preferably, UV radiation from heating system 522 is applied during theentire deposition process. Then, after the deposition of layers 27 and28 is completed, the flow of mist to the deposition chamber system 520is stopped, and the liquid coating on the substrate is preferably curedand baked in situ via UV and/or IR lamps in heating system 522.Preferably, the initial drying step takes place without breaking vacuum.This is important, because the electronic quality of the deposited layerof solid material is compromised by breaking vacuum and exposing theliquid coating to contaminants prior to drying. The substrate may alsobe removed from the deposition chamber and transferred to an annealingstation without breaking vacuum. Additional substrates may be placed inand removed from the deposition chamber system 520 without breakingvacuum.

In the above description, two liquids in two pressurized reservoirs 514and 534 and two volumetric flowrate controllers 515 and 535 were used.According to the invention three or more of each may be used. That is,the apparatus of the invention contemplates that a plurality of liquidsources and a plurality of volumetric flowrate controllers are used.

The above process has also been used to change the deposition rate ofthe precursor during deposition, preferably with a slow rate at thestart to fill in the tiniest of depressions and a faster rate after thesmaller depressions have been filled. This process simplifies thefabrication process since it avoids the use of several processing stepsto fill the depressions and then a processing step to planarize thelayer. The process has also been used with methyl ethyl ketone (MEK) asthe second liquid to alter the surface tension between exhibited by theprecursor, and thus to affect how well the precursor wets the substrate.Preferably, about a 50/50 combination of the first liquid and the MEKare used at the start to cause the precursor to wet the substrate well,and then the MEK is gradually decreased to zero, which increases thedeposition rate. This process simplifies the fabrication process byeliminating the need for a separate adhesion layer in some cases.

Now that it has been shown that the precursor viscosity can be adjustedto simplify the integrated circuit fabrication process, it is evidentthat other physical parameters of the precursor can also be adjusted toimprove the integrated circuit fabrication process. For example, thesurface tension, the density, the heat capacity, the heat ofvaporization, and the vapor pressure all can be adjusted by adjustingthe precursor, solvent and/or the initiator using the liquids discussedabove. All these properties can be adjusted for either the individualinitial metal precursors, the solvent, and/or the initiator, separately,or for the final precursor as a whole. Each of the above physicalproperties effect the performance of the precursor, the reaction of theprecursor to the deposition, drying, baking and annealing steps, and thefinal solid formed, and thus these elements of the process can becontrolled and used to advantage by adjustments made to the precursorliquids.

There has been described a novel method of fabricating integratedcircuit devices utilizing precursor liquids with selected physicalproperties, especially viscosity, in misted deposition processes. Thenovel method is useful for fabricating layers of essentially any type ofsolid material used in integrated circuits that is capable of beingformed from a liquid precursor. The novel method can be used to form,among others, gate dielectric in MOSFETs, interlayer dielectric layers,silicon dioxide insulators, and capacitor dielectric in DRAMs andFeRAMs. It should be understood that the particular embodiments shown inthe drawings and described within this specification are for purposes ofexample and should not be construed to limit the invention which will bedescribed in the claims below. Further, it is evident that those skilledin the art may now make numerous uses and modifications of the specificembodiment described, without departing from the inventive concepts. Forexample, now that the advantageous use of selectively choosing physicalproperties of precursor liquids in integrated circuit fabrication hasbeen disclosed, the method can be advantageously used in thin filmapplications other than those described. Other liquid depositionprocesses, different from those described above, may be used to formlayers of solid material. It is clear from the above that variousformulations may be used. Further, the precursors, processes andstructures can be combined with conventional processes to providevariations on the processes and devices described. It is also evidentthat the process steps recited may in some instances be performed in adifferent order, or equivalent structures and processes may besubstituted for the various structures and processes described.Consequently, the invention is to be construed as embracing each andevery novel feature and novel combination of features present in and/orpossessed by the precursors, precursor formation processes, electronicdevices, and electronic device fabrication methods and apparatusdescribed.

What is claimed is:
 1. A method of fabricating a thin film of solidmaterial having a planarized thin film surface, said method comprisingsteps of: providing a substrate having a substrate surface includingnon-planarized areas; providing a precursor liquid for forming saidlayer of solid material, said precursor liquid having a viscosity of 5centipoise or less; applying said precursor liquid to said substrate;and treating said precursor liquid on said substrate to form said thinfilm of solid material having said planarized thin film surface.
 2. Amethod as in claim 1 wherein said viscosity does not exceed 2centipoise.
 3. A method as in claim 2 wherein said viscosity is between1 centipoise and 2 centipoise.
 4. A method as in claim 1 wherein saidthin film of solid material comprises a metal oxide.
 5. A method as inclaim 4 wherein said metal oxide comprises layered superlatticematerial.
 6. A method as in claim 1 wherein said thin film of solidmaterial comprises silicon dioxide.
 7. A method as in claim 1 whereinsaid thin film of solid material comprises a silicon glass.
 8. A methodas in claim 1 wherein said precursor liquid includes a compound selectedfrom the group consisting of alkoxides, carboxylates, and combinationsthereof.
 9. A method as in claim 1 wherein said precursor liquidcomprises a metal compound.
 10. A method as in claim 9 wherein saidmetal compound is selected from the group consisting of metal2-ethylhexanoates and metal 2-methoxyethoxides.
 11. A method as in claim1 wherein said precursor liquid comprises a solvent and said solvent isselected from the group consisting of alcohols, aromatic hydrocarbons,and esters.
 12. A method as in claim 1 wherein said step of applyingcomprises spin-coating a thin film of said precursor liquid on saidsubstrate.
 13. A method as in claim 1 wherein: said step of providing asubstrate comprises placing a substrate inside an enclosed depositionchamber; and said step of applying comprises the steps of: producing amist of said precursor liquid, and flowing said mist through saiddeposition chamber to form said precursor liquid on said substrate. 14.A method of fabricating a thin film of solid material comprising stepsof: providing a substrate and a source of a first precursor liquidsuitable for forming a thin film of said solid material upon applicationto a substrate and treatment; applying said precursor liquid to saidsubstrate; subsequent to said step of applying, changing said liquidprecursor source to provide a second precursor liquid suitable forforming essentially the same solid material as said first precursorliquid but having a physical property different from said firstprecursor liquid; subsequent to said step of changing, continuing toapply said second precursor liquid having said changed physical propertydirectly onto said first precursor liquid or said solid material formedfrom said first precursor liquid; and treating said precursor liquid onsaid substrate surface to form said thin film of solid material.
 15. Amethod as in claim 14 and further including the step of drying saidprecursor liquid on said substrate between the steps of applying andcontinuing to apply.
 16. A method as in claim 14 wherein said steps ofchanging and continuing to apply are repeated at least once.
 17. Amethod as in claim 14 wherein said step of changing is performed at thesame time as said step of continuing to apply.
 18. A method as in claim17 wherein said step of changing is performed continuously for apredetermined time.
 19. A method as in claim 14 wherein said physicalproperty is viscosity.
 20. A method as in claim 14 wherein said physicalproperty is the surface tension between said precursor liquid and saidsubstrate surface.
 21. A method as in claim 14 wherein said physicalproperty is density.
 22. A method as in claim 14 wherein said physicalproperty is heat capacity.
 23. A method as in claim 14 wherein saidphysical property is the heat of vaporization.
 24. A method as in claim14 wherein said physical property is vapor pressure.
 25. A method as inclaim 14 wherein said step of applying comprises spin-coating a thinfilm of said precursor liquid on said substrate.
 26. A method as inclaim 14 wherein: said step of providing a substrate comprises placing asubstrate inside an enclosed deposition chamber; and said step ofapplying comprises the steps of: producing a mist of said precursorliquid, and flowing said mist through said deposition chamber to formsaid precursor liquid on said substrate.
 27. A method of fabricating athin film of solid material comprising the steps of: providing asubstrate; providing a source of a precursor liquid suitable for forminga thin film of said solid material; adjusting said source of a precursorliquid to change a physical property of said precursor liquid withoutessentially changing the solid material it is suitable for forming whileapplying said precursor liquid to said substrate; and treating saidprecursor liquid on said substrate surface to form said thin film ofsolid material.
 28. A method of fabricating a thin film of solidmaterial comprising steps of: providing a substrate, a first liquid, anda second liquid different from said first liquid; flowing said firstliquid through a first fluid flow controller and said second liquidthrough a second fluid flow controller; mixing said first liquid withsaid second liquid in amounts determined by said first and second fluidflow controllers to form a precursor liquid; applying said precursorliquid to said substrate; and treating said precursor liquid on saidsubstrate surface to form said thin film of solid material.
 29. A methodas in claim 28 and further including the step of adjusting said firstfluid flow controller to change the flow of said first liquid throughsaid first fluid flow controller during said step of applying.
 30. Amethod as in claim 28 wherein said at least one of said fluid flowcontrollers is a volumetric flowrate controller.
 31. A method as inclaim 28 wherein said thin film of solid material comprises a metaloxide.
 32. A method as in claim 28 wherein said thin film of solidmaterial comprises layered superlattice material.
 33. A method as inclaim 28 wherein said thin film of solid material comprises silicondioxide.
 34. A method as in claim 28 wherein said thin film of solidmaterial comprises a silicon glass.
 35. A method as in claim 28 whereinsaid precursor liquid comprises a metal compound.
 36. A method as inclaim 28 wherein said precursor liquid includes a compound selected fromthe group consisting of alkoxides, carboxylates, and combinationsthereof.
 37. A method as in claim 28 wherein said precursor liquidcomprises a solvent and said solvent is selected from the groupconsisting of alcohols, aromatic hydrocarbons, and esters.
 38. A methodas in claim 37 wherein said solvent is selected from the groupconsisting of xylenes, 2-methoxyethanol, n-butyl acetate, 1,4-dioxane,methanol and methyl ethyl ketone.
 39. A method as in claim 28 whereinsaid step of treating comprises a process selected from the groupconsisting of: exposing to vacuum, exposing to ultraviolet radiation,exposing to infrared radiation, electrical poling, drying, heating,baking, rapid thermal processing, and annealing.
 40. A method as inclaim 28 wherein said step of treating comprises maintaining asub-atmospheric pressure in a deposition chamber containing saidsubstrate.